1. Field of the Invention
The invention relates generally to a hydrostatic drive system for a vehicle, such as an industrial truck, with a hydrostatic traction drive system, a hydraulic work system and a hydraulic steering system.
2. Description of the Prior Art
On known hydrostatic drive systems for vehicles, such as for a fork-lift truck or a wheel loader, the hydrostatic drive system generally has an adjustable-delivery pump and at least one drive motor connected to the pump in a closed circuit. To provide power to the hydraulic work system, there is an additional pump with a constant delivery volume which operates in an open circuit and sucks hydraulic fluid from a reservoir and, when the hydraulic work system is not in operation, returns the hydraulic fluid to the reservoir. The steering system is supplied with hydraulic fluid by an additional pump that also has a constant delivery volume and is operated in an open circuit.
On such drive systems, therefore, a plurality of pumps, for example three, are required to supply the hydraulic energy users with hydraulic fluid. The drive system therefore takes up a great deal of space. Additionally, all the pumps are continuously driven by a drive motor, although as a rule only one or two pumps are used simultaneously during the operation of the vehicle. That results in corresponding idle losses which in turn result in a low efficiency for the drive system. The continuous operation of all the pumps also results in unnecessary wear.
Therefore, it is an object of this invention to provide a hydrostatic drive system that is compact and in which the energy utilization in the drive system is improved.